Over the year, I have a couple of diagonals and the one which is always with me, is a 1.25" Tele Vue mirror diagonal which comes with my Ranger 20 years ago!
My first diagonal was a 1.25" hybrid mirror diagonal from my C90, it converts 0.965" into 1.25" which is essential. I have no way and no experience to judge its performance.
My second one is that Tele Vue, top quality I would say. Then I bought a 2" Antares diagonal for my SCT and that's what I love, too. I owned a Zhi tong 2" mirror diagonal, too. It's cheap but with similar performance with the 2" Antares! I also owned a 1.25" Astrotech diagonal briefly, this is a dielectric model which has high reflexivity... I don't find it much better than my Tele Vue so I sell it quickly.
After that my astronomy time shrinks, and that Tele Vue aged over time. I don't find any performance drop, since it's a so slow process. But mentally I know it's aged. And now since I transformed my Ranger into 2" focuser, and my new 85ED has a 2.5" focuser, so I ordered a new one which is an Explore Scientific.
The first impression is that it comes with a huge box!
That so-call carbon fibre was only on the two side, not much help with weight saving nor performance about temperature changes. It's somewhat stylish, however.
The bottom has four screws which must be for collimation, and I believe no one would want to touch them!
I shall see if it helps on imaging... I know a diagonal must be taken away for imaging, but for the sake of convenience, I always use a diagonal for solar system imaging. I also want to go back to 2" eyepiece for visual scanning which is my prime interest in visual astronomy!
My particular sample has missed the protective ring in the 2" interface and the seller agreed to send me one later:
It would be nice if the 1.25" adapter is recessed into the 2" interface to save some back focus. My 1.25" 20mm widescan can just reach focus with this diagonal... less than 1cm remains.
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